The present invention relates to a novel sound-absorbing foam system in the form of an open-cell, mineral-organic material which can also achieve building material class B1 pursuant to DIN 1402 and is used for reducing sound levels, in particular in interior rooms. The material is characterised in that it contains lignin sulphonate.
The acoustic ratios in rooms depend heavily upon architectural factors. The parameters which determine the acoustic effect of a room can be influenced to a greater or lesser extent by a corresponding interior design. In addition to the simple noise reduction, adapting the acoustic properties of a room to its intended purpose is an important aim in room acoustics. In contrast to the outside world, the sound fields in rooms are diffuse since they are generated from direct and reflected sound. They can be controlled by correspondingly reducing the sound level. Technical sound absorbers are used for this purpose which permit targeted absorption and reflection processes.
In basic terms, technical absorbers can be split into 2 groups depending upon their mode of operation, namely resonators and porous absorbers.
In very general terms, resonators operate as acoustic spring-mass systems which have a distinct sound absorption peak. Plate resonators, Helmholtz resonators or micro-perforated absorbers are examples of such sound absorbers.
In contrast thereto, the sound energy is absorbed in porous absorbers firstly by way of friction on the pore walls where the sound energy is converted into heat energy. An open-cell structure having sufficient porosity is required for this purpose. Owing to the sound absorption produced primarily by way of dissipation, porous sound absorbers have a sound absorption spectrum which is significantly different to that of resonators. In the ideal scenario, the frequency-dependent degree of sound absorption continuously increases in an s-shaped manner at higher frequencies and asymptotically approximates a maximum value. FIG. 1 shows a typical sound absorption spectrum of a resonator and porous absorber. Porous absorbers can be constructed in a variety of ways. The material options are extremely diverse.
An improvement in the sound-absorbing properties of porous absorbers for the frequency range <1000 Hz can only be achieved by adding further sound-absorbing features. In combination with perforated plates, such a system additionally has the function of a Helmholtz resonator which means that in this case higher degrees of sound absorption can also be achieved in the lower frequency range. This is associated with an additional material outlay and further working processes.
In addition, the improvement in the sound absorption in this frequency range can also be achieved by significantly increasing the absorber mass, which is in no way desirable in many applications.
The most popular systems for sound absorption, their materials and the associated characteristics will be briefly described hereinafter:
Fibre-Containing, Porous Sound Absorbers:
Textiles:
In the simplest example, non-coated, open-cell textiles can be used as curtains. In particular lower frequencies, at which porous absorbers demonstrate low absorption behaviour owing to the system, can be reduced by leaving a specific distance with respect to a sound-reflecting wall.
Non-Woven Materials and Felts:
Nowadays, modern non-woven materials and felts for sound absorption applications have an optimum flow resistance and are generally on the market as acoustic non-woven materials or sound-absorbing, fibre insulating materials. The corresponding products can have an increased flame-resistance by means of specific flame retardants and can achieve the building material class B1 “building material having flame-resistant properties”. In order to produce acoustic non-woven materials or sound-absorbing, fibre insulating materials, native and also synthetic fibres and fibrous materials are used. Hemp, flax, reed, coconut, cotton, straw and wood or cellulose fibres as well as lamb's wool are examples for the technical use of native fibrous materials in porous sound absorbers. An overview of the native raw materials for producing sound-absorbing fibre insulating materials and their properties can be found for example in “Dämstoffe aus der heimischen Natur”, CMS Deutschland (ed.) 1997. In addition, such fibre insulating materials are also produced from the most varied synthetic fibres and fibrous materials such as for example polyester.
Glass and Mineral Substance Materials:
Sound absorbers consisting of glass or mineral fibre materials are widely used. They are produced on a large scale from fine fibre filaments which are processed to form plates or comparatively soft mats. Their bulk densities are between 40 kg/m3 and 250 kg/m3. In order to increase the stability under load, small amounts of binders are frequently added to the fibre products during the production in the fibre application process. Glass and mineral fibre plates are frequently used in acoustic ceilings. Owing to their predominantly or completely inorganic structure, they meet the building material classes A1 or A2 pursuant to DIN 4102. The binders used to produce glass and mineral fibre plates are frequently included among the phenol resins whose ecological and physiological risk potential is not insignificant.
The method described in DE 101 18 136 for producing moulded bodies from a network of mineral fibres demonstrates possibilities of obtaining glass or mineral fibre plates without such a binder by using sodium silicate solutions with subsequent sintering.
Fibre-Free, Porous Sound Absorbers:
Dispersed Solids:
Among the variety of materials of fibre-free, porous sound absorbers, dispersions of solids in the gas phase represent a large and widely used system group. In the simplest case, they firstly have a coagulation structure and can be produced by the feeding of material particles. The solid components can already be present in a porous form. Expanded clay, perlites, expanded layer minerals such as vermiculites, mineral chips, glass foam, wood, cork, cellulose or synthetic materials are examples hereof. These are used for example as loose bulk insulators in partition wall regions, as can be practically applied in structural engineering. Many of said materials can be adhered together under pressure in the filling materials using a corresponding binder. Mineral-inorganic material particles can additionally be fixed together by sintering. More recent examples for dispersed solids which are suitable as technical sound absorbers are listed hereinafter:
DE 10 2005 055 575 A1 describes by way of example filling materials consisting of ballast, expanded clay, concrete, asphalt, wood or various synthetic materials or mixtures hereof which can be bonded together to form corresponding moulded parts using suitable binders and are used as sound-absorbing components for tracks for rail vehicles.
DE 197 12 835 C3 describes sound-absorbing lightweight materials. Expanded clay, perlite or foam glass filling materials having sodium silicate are wetted, dried and then sintered to form moulded bodies having bulk densities of 150-750 kg/m3.
DE 195 39 309 C2 describes a sound protection or sound insulating material as well as a method for the production thereof, which contains fibres and which is simultaneously included in the class of dispersed solids. It is produced by way of a combination, containing a binder, of cellulose fibre filling materials and organic or inorganic secondary raw materials or mixtures hereof.
DE 195 33 564 A1 describes a sound-absorbing composite material which likewise belongs to the latter material group. Aerogel particles are combined with organic or inorganic fibre materials and are processed with water glass or melamine formaldehyde resins to form flat moulded bodies.
Foams:
Foam products are generally two-phase systems, wherein one phase is gaseous and the other phase is solid or liquid. The gaseous phase consists of fine gas bubbles which are either spherical or tetrahedral and are delimited by solid or liquid cell webs.
They can thus be split into two large groups: ball foams and tetrahedron foams. The cell webs are connected together via branch points and form a skeleton.
Foams having sound-absorbing properties are mostly open-celled. In this case, the thin walls between the delimiting webs are destroyed and the cells are connected together. As a result, the material acts as a porous absorber. The material characteristic of the cell webs in open-cell foams is extremely diverse. It ranges from metals to inorganic materials to organopolymers which nowadays represent by far the largest proportion in technical usage and are generally referred to as foamed materials. Depending upon their hardness, organopolymer foams are split into soft and hard foams. For these foams, bubble formation is mostly effected via a blowing gas which is created in situ by a chemical reaction or by a chemical compound which is dissolved in the organic matrix and boils at low temperatures or breaks down into gaseous products. In addition, foams can also be produced by the mechanical mixing-in of gases, by the polymerisation in solution under phase separation or by the use of filling materials which is dissolved away after hardening.
A numerically large proportion of the technically used open-cell organopolymer foams include those whose cellular skeleton is generated from reactive matrices such as PF, MF or PUR. Nowadays, the latter are indispensable in everyday use and in technical use. They can be produced in a comparatively simple and rapid manner as hard or soft foams having the most varied range of property profiles. Open-cell PUR foams are described many times in the literature. An overview can be found in G. Oertel, Polyurethane, Becker Braun Kunststoffhandbuch 7, Hanser Verlag, Munich 1983.
They are typically produced from isocyanate-containing compounds and polyols. Blowing gases are predominantly used to form the foam and are physically effective owing to their low boiling point. Specific blowing gas combinations consisting of physically effective blowing gases and CO2, which is produced by the chemical reaction of the isocyanate groups with water during foaming, are well known. During a reaction of water and isocyanates, in contrast to the reaction with polyols, urea groups are produced in addition to CO2 and contribute to the formation of the cellular skeleton. DD 292 467 contains such a method for the production of elastic and open-cell polyurethane soft foam substances which are obtained from isocyanates and polyether polyols in the presence of water and organic blowing agents.
In view of the discussions regarding global warming, in recent times increased “water-blown” polyurethane foam substances have been developed. The foam is formed, without the aid of physical blowing gases, exclusively by the blowing gas CO2 which is produced by way of the chemical reaction of the isocyanate groups with water. DE 199 05 089 A1 describes by way of example fine-cell, water-blown polyurethane hard foam substances with >85% open cells. This is obtained by converting polyisocyanates with a polyol component present as an emulsion.
Opening the cell walls of water-blown foams by way of mechanical post-treatment (milling) is effected solely on, in particular, soft foams owing to the risk of damage to the foam skeleton. So-called cell openers are frequently used. These weaken the cell walls which are then destroyed at the points of weakness owing to the increasing excess pressure in the cells during the bubble growth when forming the foam. This weakening can be produced for example by solids or other interface-active substances. If the cellular skeleton is generated from reactive components, then further reactants can be added which form phases, which are non-soluble with the surroundings in an early stage during the foaming process, and thereby weaken the cell walls. Specifically in the case of polyurethane foams, cell opening can additionally be supported by way of water vapour which is available as an additional gas amount and becomes effective at an inner temperature of 100° C. (cf. J. H. Saunders, Fundamentals of Foam Formation in D. Klempner, K. C. Handbook of Polymeric Foams and Foam Technology, Hanser Verlag Munich 1991, pg. 12). DE 691 31 452 T2 describes an energy-absorbing polyurethane foam which can be produced in this manner.
The method illustrated in DE 10 2004 046 172 B4 for producing an open-cell polyurethane foam without skin formation also describes the use of water vapour for supporting the opening of the cells. A great deal of technical importance is placed upon the use of additives which are interface-active and weaken the cell walls at their thin locations such that a passage is created during the foaming process. It can be seen from the multitude of solution proposals which can be derived from the current literature in this respect that narrow system limits exist for the desired effect of interface-active, cell-opening additives. Even small changes result in partially serious damage to the cellular skeleton. The most important recent literature references are listed hereinafter in brief:
For example, DE 43 03 809 C2 describes open-cell PUR hard foams consisting of so-called 1-component systems by adding specific liquid polyolefins in amounts of 0.1-3.0% by weight.
FR-A-1 461 357 likewise proposes the use of hydrocarbons for opening cells. U.S. Pat. No. 4,826,383 and U.S. Pat. No. 4,863,975 describes oxynitrate salts as effective cell openers for comparable systems. The use of siloxanes and polysiloxane-polyoxyalkylene block polymers for opening cells is also known, e.g., in DE-A-39 28 867.
In contrast, DE 43 18 120 C5 contains a method for producing open-cell PUR soft foams by using specific polyoxypropylene-polyoxyethylene-polyols which are to have the cell-opening effect.
DE-A-1 2 48 286 and U.S. Pat. No. 4,596,665 describe low-molecular polyglycols or polyoxyalkylene oxides which are to facilitate the open-cell PUR soft foams. DE 100 09 649 and DE 103 36 938 describe open-cell polyurethane hard foams which are obtained by way of the use of polyol components consisting of esterification products of glycerine and castor oil or polyether alcohols.
However, sound energy can also be converted into other energy forms by way of relaxation processes in the skeleton substance. Polymer-organic foams, whose polymer skeleton is set such that large relaxation processes can occur in the corresponding region, have significant sound-absorbing properties in that location. The impinging sound waves cause the skeleton to vibrate. Owing to the relaxation processes taking place, the vibration energy is converted in particular into heat (cf. H. Oberst, Werkstoffe mit extrem hoher innerer Dämpfung in Acustica, 1955, pg. 141-pg. 151). Laid-open document 28 35 329 contains for example a polyurethane foam for the purposes of reducing noise. By suitably adjusting the relaxation processes, high sound absorptions in the range of <300 Hz at a sample thickness of 30 mm were achieved. DE 199 24 802 A1 describes a method for the “Herstellung von schalldämpfenden and energieabsorbierenden Polyurethanschäumen [production of sound-insulating and energy-absorbing polyurethane foams]” based on specific polyether polyols and modified polyisocyanates. The loss factor tan δ of the material is, according to the statements, >0.3. The use of viscoelastic substances for sound insulation is also described in structural applications. DE 39 42 760 A1 shows, for example, the use of polyvinyl butyral as a viscoelastic layer in garage door sheets. DE 698 20 676 T2 relates to a vibration-insulating composite material having an inner viscoelastic adhesive layer. DE 692 07 437 T2 describes a sound-insulating sandwich material and a method for the production thereof. A sound-insulating elastomer PUR adhesive having a mechanical loss factor of tan δ 0.3-0.4 in the frequency range of 200 Hz-2000 Hz is used.
Finally, EP 1186 630 B1 contains an organic-hybrid insulating material which contains an insulation improver which is formed from a mixture of specific phenolic compounds.
Since the polyol component also has a substantial influence on the physical properties of the formed cellular skeleton, it is indispensable for most formulations.
Purely water-blown formulations based on isocyanate-containing compounds without further organic reactants such as polyols are therefore extremely rare. Laid-open document DE 25 24 191 contains the description of a highly-filled polyurea foam material which can be produced from polyisocyanates, water, catalysts, stabilisers and finely dispersed filling materials, but which does not have open cells.
DE 39 09 083 describes a gypsum foam material having a porous structure as well as a method for the production thereof for sound and heat insulation. A gypsum-water suspension is mixed with an MDI prepolymer without further reactants in the presence of a surfactant and is foamed to form moulded bodies.
Together with the description of a method for producing a halogen-free and filling material-containing, flame resistant polyurea foam, DE 25 241 91 A1 includes the further development of the gypsum foam material disclosed in DE 39 09 083. The bulk density of the foam material which can be achieved can be lowered considerably. In addition, by adding larger amounts of ammonium polyphosphate, an increased flame resistance of the foam is to be achieved.
However, it has been shown in practice that it is difficult to obtain the foams produced in accordance with DE 25 241 91 A1 with open cells. In order to achieve the minimum acoustic properties as set forth in accordance with its purpose of use, the foam bodies must be subsequently mechanically processed by milling or needling. A portion of the foam cells can be opened hereby. Simultaneously, the foam material loses rigidity by way of this working step. This is a disadvantage for many applications and means that structural auxiliary solutions are required. In addition, a greater extent of broadband absorption capabilities can be achieved only by way of a combination with perforated plates.